Seeing at a distance with multicore fibers

Images and videos provide a wealth of information for people in production and life.Although most digital information is transmitted via optical fiber,the image acquisition and transmission processes still rely heavily on electronic circuits.The development of all-optical transmission networks and optical computing frameworks has pointed to the direction for the next generation of data transmission and information processing.Here,we propose a high-speed,low-cost,multiplexed parallel and one-piece all-fiber architecture for image acquisition,encoding,and transmission,called the Multicore Fiber Acquisition and Transmission Image System(MFAT).Based on different spatial and modal channels of the multicore fiber,fiber-coupled self-encoding,and digital aperture decoding technology,scenes can be observed directly from up to 1 km away.The expansion of capacity provides the possibility of parallel coded transmission of multimodal high-quality data.MFAT requires no additional signal transmitting and receiving equipment.The all-fiber processing saves the time traditionally spent on signal conversion and image pre-processing(compression,encoding,and modulation).Additionally,it provides an effective solution for 2D information acquisition and transmission tasks in extreme environments such as high temperatures and electromagnetic interference.

Advances in femtosecond laser direct writing of fiber Bragg gratings in multicore fibers:technology,sensor and laser applications

In this article,we review recent advances in the technology of writing fiber Bragg gratings(FBGs)in selected cores of multicore fibers(MCFs)by using femtosecond laser pulses.The writing technology of such a key element as the FBG opens up wide opportunities for the creation of next generation fiber lasers and sensors based on MCFs.The advantages of the technology are shown by using the examples of 3D shape sensors,acoustic emission sensors with spatially multiplexed channels,as well as multicore fiber Raman lasers.

Distributed multicore fiber sensors

Multicore fiber(MCF)which contains more than one core in a single fiber cladding has attracted ever increasing attention for application in optical sensing systems owing to its unique capability of independent light transmission in multiple spatial channels.Different from the situation in standard single mode fiber(SMF),the fiber bending gives rise to tangential strain in off-center cores,and this unique feature has been employed for directional bending and shape sensing,where strain measurement is achieved by using either fiber Bragg gratings(FBGs),optical frequency-domain reflectometry(OFDR)or Brillouin distributed sensing technique.On the other hand,the parallel spatial cores enable space-division multiplexed(SDM)system configuration that allows for the multiplexing of multiple distributed sensing techniques.As a result,multi-parameter sensing or performance enhanced sensing can be achieved by using MCF.In this paper,we review the research progress in MCF based distributed fiber sensors.Brief introductions of MCF and the multiplexing/de-multiplexing methods are presented.The bending sensitivity of off-center cores is analyzed.Curvature and shape sensing,as well as various SDM distributed sensing using MCF are summarized,and the working principles of diverse MCF sensors are discussed.Finally,we present the challenges and prospects of MCF for distributed sensing applications.

Theoretical investigation of core mode cut-off condition for tapered multicore fiber

Core mode cutoff is a useful concept not only for a tapered single-core fiber(SCF) but also for a tapered multicore fiber(MCF) to realize cladding mode transmission. In this paper, cut-off conditions of either core mode for tapered SCFs or supermodes for MCFs are theoretically investigated. Rigorous analytical formulas are derived for the modes of SCF by a three-layer waveguide model, and an approximation formula of the cut-off condition is given for the LP01 mode. The supermodes of MCFs are analyzed by the coupling mode theory, and the cut-off condition is calculated by a numerical method. Simulation results show that the in-phase supermode of MCFs has a similar cut-off condition with that of SCF. Based on this property, a convenient approximate formula is given to estimate the cut-off condition of the in-phase supermode for tapered MCFs.

C-band real-time 400/300 Gb/s OOK bidirectional interconnection over 20 km multicore fibers

With the increasing capacity demands fueled by the emerging heterogeneous and bandwidth-intensive applica- tions, such as cloud computing, 4K/8K videos, and virtual reality （VR）, the need for low-cost, power-efficient, and high-density short-reach optical interconnects operating at 100 Gb/s and beyond has drawn significant research efforts.

High‑Accuracy 3D Shape Sensor Based on Anti‑Twist Packaged High Uniform Multicore Fiber FBGs

Improving the accuracy of shape sensors based on multicore fibers(MCFs)is challenging but of great importance for real-time 3D shape detection,especially in visually inaccessible areas.In this work,a novel approach is proposed to improve MCF shape sensor accuracy using an ultraviolet transparent liquid mediated fiber Bragg grating(FBG)inscription technique and a twist-isolating packaging method.A newly developed UV index matching liquid(UV-IML)is used to generate uniform light field at all the MCF cores,enabling FBG inscription with high accuracy.Additionally,a new stress fully released(SFR)packaging method is implemented to isolate the sensor from any external twists.The MCF shape sensor shows a maximum relative error of only 3.33%and the lowest reported relative sensitivity error of 1.11%cm^(-1).Moreover,a real-time 3D shape sensing system with a response frequency larger than 30 Hz is constructed using the unique MCF shape sensor.The highly accurate real-time 3D shape sensing results indicate potential applications for in vivo shape estimation of endoscopies and soft robots.

Stable optical vortices in nonlinear multicore fibers

The multicore fiber(MCF)is a physical system of high practical importance.In addition to standard exploitation,MCFs may support discrete vortices that carry orbital angular momentum suitable for spatial-division multiplexing in high-capacity fiber-optic communication systems.These discrete vortices may also be attractive for high-power laser applications.We present the conditions of existence,stability,and coherent propagation of such optical vortices for two practical MCF designs.Through optimization,we found stable discrete vortices that were capable of transferring high coherent power through the MCF.

Performance Analysis of Optical Transmission Based on Seven Cores Fiber

This essay designed a kind of new seven-core fiber with lower crosstalk and loss, and made space division multiplexing transmission experiment based on this seven-core fiber. It is known that crosstalk has the most serious influence in multicore fiber transmission process. Before the experiment, the affecting factors of fiber crosstalk were analyzed through simulation, such as core space, bending radius, and fiber length. Combined with the simulation analysis, the design scheme of multicore fiber with low crosstalk was obtained. Before the fiber design, various factors of influence crosstalk such as the core- to-core distance, bending radius, fiber length and so on. Based on the simulation analysis, conclusion has made on the design scheme of multi-core optimal fiber with low crosstalk. The space division multiplexing and wavelength division multiplexing technology, was adopted to conduct seven-core optical fiber transmission of 58.7kin.The crosstalk of adjacent core was suppressed to as low as 45dB / km, the attenuation of inner core was 0.24dB/ km, the outer cores＇ 0.32dB/km. Different bit error rate （BER） performances were also studied under different conditions, through reasonably designing the system to reduce the error rate, improve the performance of the system, and realize long distance and large capacity transmission with fiber.

Multicore fiber with thermally expanded cores for increased collection efficiency in endoscopic imaging

Fiber-based endoscopes are promising for minimally invasive in vivo biomedical diagnostics.Multicore fibers offer high resolution imaging.However,to avoid image deterioration induced by inter-core coupling,significant spacing between cores is required,which limits the active image guiding area of the fiber.Thus,they suffer from low light collection efficiency and decreased signal-to-noise ratio.In this paper,we present a method to increase the collection efficiency by thermally expanding the cores at the facet of a multicore fiber.This expansion is based on the diffusion of doping material of the cores,thus the fiber’s original outer diameter is preserved.By enlarging the core diameter by a factor of 2.8,we increase the intensity of the transmitted light by a factor of up to 2.3.This results in a signal-to-noise ratio increase by a factor of up to 4.6 and significant improvement in the image contrast.The improvement increases with increasing working distance but is already prominent for as small working distance as 0.5 mm.The feasibility of the method is proved experimentally by lensless single-shot imaging of a test chart and incoherent light reflected from clusters of microbeads.The demonstrated approach is an important tool especially in imaging of biological specimens,for which phototoxicity must be avoided,and therefore,high collection efficiency is required.

Low-insertion-loss femtosecond laser-inscribed three-dimensional high-density mux/demux devices

Recently,transmitting diverse signals in different cores of a multicore fiber(MCF)has greatly improved the communication capacity of a single fiber.In such an MCF-based communication system,mux/demux devices with broad bandwidth are of great significance.In this work,we design and fabricate a 19-channel mux/demux device based on femtosecond laser direct writing.The fabricated mux/demux device possesses an average insertion loss of 0.88 dB and intercore crosstalk of no more than−29.1 dB.Moreover,the fabricated mux/demux device features a broad bandwidth across the C+L band.Such a mux/demux device enables low-loss 19-core fiber(de)multiplexing over the whole C+L band,showing a convincing potential value in wavelength-space division multiplexing applications.In addition,a 19-core fiber fan-in/fan-out system is also established based on a pair of mux/demux devices in this work.

Highly sensitive torsion sensor based on Mach–Zehnder interference in helical seven-core fiber taper

We propose a high-sensitivity bidirectional torsion sensor using a helical seven-core fiber taper embedded in multimode fiber(MHSTM).Sensors with different taper waists and helical pitches are fabricated,and their transmission spectra are obtained and analyzed.The waist and length of the sandwiched seven-core fiber are finally determined to be 68 μm and3 mm,respectively.The experimental results show that the clockwise and counterclockwise torsion sensitivities of the proposed sensor are 2.253 nm/(rad/m) and-1.123 nm/(rad/m),respectively.When tapered waist diameter reduces to48 μm,a superior torsion sensitivity of 5.391 nm/(rad/m) in the range of 0-4.24 nm/(rad/m) is obtained,which is 46 times as large as the traditional helical seven-core fiber structure.In addition,the MHSTM structure is also relatively stable to temperature variations.

Single-shot 3D incoherent imaging with diffuser endoscopy

Minimally invasive endoscopy offers a high potential for biomedical imaging applications.However,conventional fiberoptic endoscopes require lens systems which are not suitable for real-time 3D imaging.Instead,a diffuser is utilized for passively encoding incoherent 3D objects into 2D speckle patterns.Neural networks are employed for fast computational image reconstruction beyond the optical memory effect.In this paper,we demonstrate single-shot 3D incoherent fiber imaging with keyhole access at video rate.Applying the diffuser fiber endoscope for fluorescence imaging is promising for in vivo deep brain diagnostics with cellular resolution.

Lensless fiber endomicroscopy in biomedicine

Lensless fiber endomicroscopy,an emergent paradigm shift for minimally-invasive microscopic optical imaging and targeted light delivery,holds transformative potential,especially in biomedicine.Leveraging holographic detection and physical or computational wavefront correction,it enables three-dimensional imaging in an unprecedentedly small footprint,which is crucial for various applications such as brain surgery.This perspective reviews the recent breakthroughs,highlighting potential emerging applications,and pinpointing gaps between innovation and real-world applications.As the research in this realm accelerates,the novel breakthroughs and existing frontiers highlighted in this perspective can be used as guidelines for researchers joining this exciting domain.